Method of electroplating zinc



1 Sept 8, 1953 A. E. cHEsTER` 2,651,610

METHOD OF ELECTROPLATING ZINC 'Filed July 17, 1950 220- 440KHC[lavan/iva Hura- 7AM/umana 3 ow 1647465 mwf ufasrar acide-Mmm HH 1 raafo Q.. ...T/Mt.

Patented Sept. 8, 1953 METHOD F ELECTROPLATING ZINC Allan E. Chester,Highland Park, Ill., assigner to Poor & Company, Chicago, Ill., acorporation of Delaware Application July 17, 1950, Serial No. 174,242

2 Claims. 1

This invention relates to a new and improved method of electroplating,and more particularly to a method of electrodepositing metals from aplating bath in a new and improved manner adapted to producesubstantially uniform distribution on articles or objects ofconfigurations such that the ordinary type of electroplating causesuneven distribution of the electrodeposited metal.

It is well known that flat objects, when electroplated in the usual wayin an electroplating bat tend to build up a higher concentration ofmetal near the edges than in the middle. This is true to some extent inobjects of other configurations and various devices have been proposedin order to overcome this diiiiculty.

One of the objects of the present invention is to provide a new andimproved method for overcoming the difficulty mentioned above and forelectrodepositing metals from plating baths in a uniform manner.

Another object of the invention is to provide a new and improved methodfor electrodepositing metals on flat sheets or other flat-objects in amanner such that substantial uniformity of distribution is obtained.

A still further object is the provision of a new and improved method forcontrolling the distribution of a metal from a plating bath.

Other features and advantages of the invention Will appear hereinafterby reference to the following description thereof, in conjunction with vthe accompanying drawings in which:

Fig. 1 illustrates diagrammatically one form of apparatus suitable forthe practice of the invention;

Fig. 2 illustrates graphically one of the limits of the wave form of theplating current; and

Fig. 3 illustrates another limit of thewave form of the plating currentin accordance with the preferred practice of the invention.

Generally stated, the invention involves a new and improved method ofcontrolling the distribution of zinc deposited on an electricallyconducting surface from a plating bath by superimposing alternatingcurrent on direct current within certain predetermined limits. Theinvention is especially applicable to electroplating operations in whichthe work or article is so shaped that the electrodeposited metal buildsup in some areas more than others. The plating current for the purposeof this invention may be described as an asymmetric alternating currentin which the negative peak value of the wave form varies from zero (asillustrated in Fig. 2) to -2/3 of the positive peak value (asillustrated in Fig. 3).

Referring to Fig. 1, the apparatus illustrated comprises a suitablereceptacle I containing the plating bath, provided with one or moreelectrodes, generally indicated at 2, and a work piece or conductingsurface to be plated, generally indicated at 3. As will be understood,the electrode 2 forms the anode and the work piece 3 the cathode.

The anode 2 is connected by suitable conductors to the secondary -4 of apower transformer having a core 5 and a primary 6. The primary 6 isconnected to a regulating auto-transformer, generally illustrated at 1,which in turn is connected to a suitable source of electrical energy,such as a 220 or 440 volt alternating current. Direct current issupplied by means of a low voltage current generator, generally shown at8, one side of which passes to the work piece 3 through a rheostat 9,and the other side of which is connected to the secondary 4 in themanner shown.

In Figs. 2 and 3 the Wave form of the current is graphically illustratedin a conventional manner,-

the horizontal axis (X--X) representing time and the vertical axisamplitude. The peak values are calculated from the equationE=1.414\/E2A. C. (R. M. S.) +E2 D. C.

y(Y1) falls below the X-axis and is a different value from the positivepeak value (Y in Fig. 2, Z in Fig. 3) represents the maximum negativevalue (Z1) which may be employed for the purpose of this invention. Anywave form similar to Fig. 2 in which the negative peak value is positiveor above the X-axis may be described as pulsating direct current.

In a broader sense, the apparatus described and the hook-up of thecomponent parts is believed to represent a new and improved system forthe electrodeposition of metals by the superimposition of an alternatingcurrent on a direct current, irrespective of whether the ratios ofalternating current to direct current are within the limits preferredfor uniform distribution. Thus, this type of apparatus may be employedin plating articles of any configuration with other ratios ofalternating current to direct current where the distribution of theelectrodeposited metal is not an important factor. This type ofapparatus is suitable, therefore, for electroplating operations by thesuperimposition Aof alternating current on direct current in such amanner as to form a pulsating direct current.

'I'he method of controlling distribution of the electrodeposited metalas described herein is especially suited for the formation of very thin,substantially uniform films or coatings of a metal on an electricallyconductive surface. For example, it has been found that this inventionis especially adapted for forming coatings of Zinc on steel or cast ironobjects in which the thickness of the coating is within the range of.000005 inch to .00005 inch. Steel or cast iron objects, when platedwith zinc in accordance with the invention to the thickness described,are especially suitable for the addition of coatings of vitreousenamels, as described and claimed in my copending application Serial No.555,953 filed September 27, 1944, now Patent No. 2,615,836. Thethickness and uniformity of the plate is an important factor because ithas been found that if the electrodeposited plate is insumcient, thedesired bonding action will not occur. The same is true if the thicknessof the coating is too great. Thus, this invention makes possible new andimproved results in the art of enameling, as well as new and improvedresults in controlling the distribution of metals by electrodeposition.

The invention will be further illustrated but is not limited by thefollowing example:

Example A typical zinc plating bath which may be employed has thefollowing composition:

Ounces per gallon ZnSOrJYI-IZO 48 NH4Cl 4 NaCzHsOaSHbO (sodium acetate)2 l-IzSOrx 0.4

- The bath may be puriiied by electrolyzing it as described inTransactions of Electrochemical Society, vol. 80, (1941) pages 391 and392. The ratios of alternating to direct current may be varied in orderto obtain the optimum wave form in the manner previously described.

It will be understood that the invention is susceptible vto somevariation and modication in the manner of its practicalv application.

The ratios of alternating to direct current which are preferablyemployed in accordance with the invention in order to obtain the desiredWave form, are about 2-4 volts (root mean square) A. C.: 2.5 volts D.C., preferably at current densities from 2 to 15 amperes per squarefoot. Alternating currents of different frequencies may be used, but itis preferable to employ either a 25 cycle or a 60 cycle current. Theplating time will vary depending upon the amount of metal which it isdesired to deposit. Thus, for thicknesses of the type previouslymentioned for vitreous enamel work, a plating time from 8 seconds to11/2 minutes is sufficient. It will be understood that higher currentdensities may be used, and that the alternating and direct currentvoltages may be larger or smaller, provided that the ratio produces apredetermined wave form of the type previously described. Thus, if thedirect current is 0.5 volt, the alternating current would becorrespondingly reduced in accordance with the foregoing ratio. If thedirect current is 10 volts, the alternating current is increased in theforegoing ratio.

The plating current will be governed by the surface area of the articlebeing plated, the size of the bath, power facilities and other factors 4which will be recognized by those skilled in the art.

The expression zinc plating bat is intended to include plating baths ofthe type described in the example Where other metals or salts thereofare present in minor proportion as well as those in which such metals orsalts are absent.

The invention provides a new and improved method for controlling thedeposition of zinc on electrically conducting surfaces. It is especiallyuseful in applying even, uniform coatings to articles having aconfiguration in which, by ordinary direct current plating methods, thedeposited metal tends to build up in some areas more than others. Thus,it is especially important for electroplating fiat surfaces when a thinplate of substantially uniform thickness is desired.

This application is a continuation-in-part of my copending applicationSerial N o. 555,954, filed September 27, 1944, now U. S. Patent No.2,515,192.

The invention is hereby claimed as follows:

1. In a method of controlling the electrodeposition of zinc from an acidzinc sulphate plating bath onto an electrically conducting object of atype in which the zinc would normally be deposited in greaterconcentrations in some areas than in others by a direct plating current;the step which comprises electrodepositing the zinc by subjecting saidobject in an acid plating bath to a plating current having an asymmetricalternating wave form comprising an alternating current superimposed ona direct current in the ratio of 2-4 volts R. M. S. to 2.5 volts D. C.,regardless of the speciiic current values, at current densities fromabout 2 to about 15 amperes per square foot.

2. In a method of controlling the electrodeposition of zinc from an acidzinc sulfate plating bath onto a work piece of a type in which the zincis normally deposited in greater concentrations in some areas of thework than in others by direct current plating; the step which compriseselectrodepositing zinc on said work piece from an acid bath by asuperimposed alternating-direct plating current comprising analternating current to direct current ratio from about 2 to about 4volts R. M. S. alternating current having a frequency within the limitsfrom about 25 cycles to about 60 cycles to about 2.5 volts D. C.,regardless of the specific current values, at current densities fromabout 2 to about 15 amperes per square foot for a'period of time withinthe range from 8 seconds to about 11/2` minutes.

ALLAN E. CHESTER.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,918,605 Jones July 18, 1933 2,063,760 Schulen Dec. 8, 19362,443,599 Chester June 22, 1948 2,515,192 Chester July 18, 1950 OTHERREFERENCES vol. 18

1. IN A METHOD OF CONTROLLING THE ELECTRODEPOSITION OF ZINC FROM AN ACIDZINC SULPHATE PLATING BATH ONTO AN ELECTRICALLY CONDUCTING OBJECT OF ATYPE IN WHICH THE ZINC WOULD NORMALLY BE DEPOSITED IN GREATERCONCENTRATIONS IN SOME AREAS THAN IN OTHERS BY A DIRECT PLATING CURRENT;THE STEP WHICH COMPRISES ELECTRODEPOSITING THE ZINC BY SUBJECTING SAIDOBJECT IN AN ACID PLATING BATH TO A PLATING CURRENT HAVING AN ASYMMETRICALTERNATING WAVE FROM COMPRISING AN ALTERNATING CURRENT SUPERIMPOSED ONA DIRECT CURRENT IN THE RATIO OF 2-4 VOLTS R. M. S. TO 2.5 VOLTS D. C.,REGARDLESS OF THE SPECIFIC CURRENT VALUES, AT CURRENT DENSITIES FROMABOUT 3 TO ABOUT 15 AMPERES PER SQUARE FOOT.